Look to see if the caps are worn. Look to see if the seals are still in place, if not some of the bearings may be broke and worn. Look to see if the joint is loose. Look for damage such as rock rash, cracks, anything that doesn't look normal after wiping away any dirt. If you find any of the above you should replace that joint. Also check the straps and seats of the joints.

I tried to find a u-joint to take pics of so I could show you but I can't find them right now, buried.

The caps are the end of the joint, they fit on normally over needle bearings. The seals hold in grease and keep out dirt and are located between the cap and the body of the joint.